1. Field of the Invention
The present invention relates to a thermal processing apparatus and a thermal processing method of irradiating a semiconductor wafer or a glass substrate (hereinafter simply referred to as “substrate”) with a flash thereby thermally processing the substrate.
2. Description of the Background Art
In general, a thermal processing apparatus such as a lamp annealing apparatus employing a halogen lamp is used in an ion activation step for a semiconductor wafer subsequent to an ion implantation step. This thermal processing apparatus heats (anneals) the semiconductor wafer to a temperature of about 1000° C. to 1100° C., for example, thereby executing ion activation of the semiconductor wafer. The thermal processing apparatus increases the temperature of the substrate at a speed of about several 100 degrees per second through the energy of light emitted from the halogen lamp.
However, it has been proved that the profile of the ions implanted into the semiconductor wafer is rounded, i.e., the ions are thermally diffused also when ion activation of the semiconductor wafer is executed through the thermal processing apparatus increasing the temperature of the substrate at the speed of about several 100 degrees per second. In this case, the ions are diffused even if the same are implanted into the surface of the semiconductor wafer in high concentration, and hence the ions must be implanted disadvantageously beyond necessity.
In order to solve the aforementioned problem, there has been proposed a technique of irradiating the surface of the semiconductor wafer with a flash emitted from a xenon flash lamp or the like thereby increasing the temperature of only the surface of the semiconductor wafer subjected to ion implantation in an extremely short time of not more than several milliseconds. When the surface of the semiconductor wafer is heated with the flash emitted from the xenon flash lamp in an extremely short time, there is no sufficient time for diffusing the ions and hence only ion activation can be executed without rounding the profile of the ions implanted into the semiconductor wafer.
When such a thermal processing apparatus is provided with a plurality of xenon flash lamps, however, a heat gradient corresponding to the light intensity gradient of the xenon flash lamps takes place on the substrate, to disadvantageously result in heterogeneous thermal processing of the substrate.
A general thermal processing apparatus using no flash lamps can solve this problem of the heat gradient by rotating the thermally processed substrate. In the thermal processing apparatus employing the flash lamps, however, the time for emitting flashes is so short that the problem of the heat gradient cannot be solved by rotating the substrate.
Also in the thermal processing apparatus employing xenon flash lamps, process conditions must be changed for obtaining optimum conditions in the process, similarly to the conventional lamp annealing apparatus. In the thermal processing apparatus employing photoirradiation, one of the most important process conditions is the intensity of irradiation. The general lamp annealing apparatus employing a halogen lamp can relatively readily adjust the intensity of irradiation by controlling power supplied to the lamp.
In the case of the thermal processing apparatus employing xenon flash lamps, however, a method of adjusting the intensity of irradiation by changing a main discharge voltage applied to the lamps results in various demerits. When the main discharge voltage applied to the xenon flash lamps is changed, the waveforms of the quantity of emission and the time are changed while spectral distribution varies with the change of the discharge voltage. In other words, not only the intensity of irradiation but also optical characteristics of the flashes vary when the main discharge voltage applied to the lamps is simply changed, to result in disintegration of correlation between the intensity of irradiation and the wafer temperature, leading to extreme difficulty of temperature control. When the main discharge voltage applied to the xenon flash lamps is changed, degradation characteristics of the lamps are also disadvantageously varied.